1. Field of the Invention
The invention relates to magnetic recording of data and more particularly to pre-compensation in writing of data to a magnetic medium.
2. Background Information
In magnetic recording, data is written on or read from one or more data tracks of a magnetic storage medium such as a hard disk. The data tracks generally form concentric rings on the surfaces of each of plural hard disks that constitute the magnetic recording device. When writing to such a track, the disk is rotated at predetermined speed, and electrical signals applied to a magnetic read/write head floating over the track are converted to magnetic transitions on the track. The magnetic transitions represent digital data encoded so that each transition may correspond to a ONE bit value and the absence of a transition may correspond to a ZERO bit value as in a “non return to zero inverted” (NRZI) encoding.
To obtain high density recording, magnetic transitions representing data patterns are closely packed on the hard disk magnetic medium. Each transition or absence of a transition of the recording bit sequence is located in a window in which a flux reversal may occur. Such closely packed data bits influence each other so that non-linear magnetic shifting of transitions and bit interference are likely to occur during recording. As a result, the reading of the high density recorded data pattern may be adversely affected. In an example, any device mismatch in a high data rate write data path causes positive/negative transition skew known as pulse pairing so that writing of a single tone square wave can result in a write transition spacing that has other than a 50% duty cycle.
When writing to a high density magnetic recording channel, it is therefore necessary to adjust the position of transitions in the data stream to correct for the influence of nearby transitions so that transitions in the recovered data stream are evenly placed. Such precompensation of data being recorded is accomplished by changing the timing of the magnetic flux reversal in a clock period to offset the magnetic transition shift and interference effects of adjacent and nearby bits. In this way, the transition shift of a bit due to the pattern of preceding and/or succeeding bits is anticipated and the bit recording time is changed to compensate for the magnetic transition shift due to the effects of surrounding bits.
In one type of known precompensation exemplified by the disclosure of U.S. Pat. No. 4,878,028, issued Oct. 31, 1989 to Y. C. Wang, a precompensation delay arrangement for writing data to disks has plural delay information elements, each providing a fixed delay that is a percentage of the bit cell time period according to the data bit pattern. In the recording of the data pattern, one of the delay information elements is selected to provide a preset delay (early, nominal, or late) for recording a present data bit according to the preceding data bit pattern.
In another type of precompensation exemplified by U.S. Pat. No. 6,133,861, issued Oct. 17, 2000 to G. Jusuf et al. and assigned to the same assignee, each of multiple decoders is supplied with a selectably variable version of a master clock. The delayed versions of the master clock are stably produced by delay elements using delay-locked feedback loops. Each of the delay elements is set at a different fixed delay and provides a differently delayed clock signal to a decoding circuit to which data to be recorded is supplied. A selection circuit selects one of the data decoding circuits responsive to the pattern of the preceding data bits.
In the aforementioned and other prior art arrangements, the timing of recording of each data bit is shifted to precompensate for the preceding data bit pattern. It is, however, required to provide 2N delay altering elements each having a different delay to provide precompensation for a data pattern of N data bits. Accordingly, while only 4 delay altering elements generating different delays are needed for precompensation of a 2 data bit pattern, the number of required delay defining elements increases as the data bit pattern is increased. Eight delay altering elements are needed for precompensation based on a three data bit pattern and 16 delay altering elements are needed for precompensation based on a 4 data bit pattern. The increased number of delay altering elements to precompensate for larger data bit patterns adds to the cost and the complexity of magnetic recording system. Accordingly, It is desirable to provide a precompensation arrangement which utilizes a predetermined number of delay elements which is independent of the size of data bit patterns used for precompensation.